U.S. patent number 6,549,267 [Application Number 09/455,271] was granted by the patent office on 2003-04-15 for pulse-width extending optical systems, projection-exposure apparatus comprising same, and manufacturing methods using same.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Yuji Kudo.
United States Patent |
6,549,267 |
Kudo |
April 15, 2003 |
Pulse-width extending optical systems, projection-exposure
apparatus comprising same, and manufacturing methods using same
Abstract
This invention pertains to systems for extending the pulse
length of pulsed sources of optical radiation. These systems reduce
peak optical pulse power without reducing average optical power.
The pulse-width extending systems split optical pulses into pulse
portions, introduce relative delays among the pulse portions, and
then redirect the pulse portions (or portions thereof) along a
common axis. Such pulse-width extending systems are especially
useful in projection-exposure apparatus for the manufacture of
semiconductor devices where short wavelength, high power optical
sources tend to damage optical components.
Inventors: |
Kudo; Yuji (Kawasaki,
JP) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
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Family
ID: |
26401031 |
Appl.
No.: |
09/455,271 |
Filed: |
December 6, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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802951 |
Feb 21, 1997 |
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Foreign Application Priority Data
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Feb 22, 1996 [JP] |
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8-059970 |
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Current U.S.
Class: |
355/53; 355/77;
372/700; 372/25 |
Current CPC
Class: |
G03B
27/42 (20130101); G03F 7/7055 (20130101); Y10S
372/70 (20130101); H01S 3/0057 (20130101) |
Current International
Class: |
G03B
27/42 (20060101); G03F 7/20 (20060101); H01S
3/00 (20060101); G03B 027/42 (); G03B 027/32 ();
H01S 003/10 (); H01S 003/00 () |
Field of
Search: |
;355/53,44,45,77
;353/122 ;362/3,268 ;430/30 ;250/492.2 ;385/14,16
;372/22,25,700 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1-198759 |
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Aug 1989 |
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JP |
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1-287924 |
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Nov 1989 |
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JP |
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6-29177 |
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Feb 1994 |
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JP |
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2-2590530 |
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Dec 1996 |
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JP |
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9-17725 |
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Jan 1997 |
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JP |
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Primary Examiner: Adams; Russell
Assistant Examiner: Kim; Peter
Attorney, Agent or Firm: Oliff & Berridge, PLC
Parent Case Text
This is a continuation of U.S. application Ser. No. 08/802,951,
filed Feb. 21, 1997, which is incorporated herein by reference.
Claims
What is claimed is:
1. A projection-exposure apparatus for projecting a pattern formed
on a mask onto a sensitized substrate, the apparatus comprising:
(a) a source of radiation operable to emit a pulse of radiation
having a peak power; (b) a pulse-width extending optical system for
extending a duration of the pulse emitted by the source of
radiation whereby the peak power is reduced, the pulse-width
extending optical system comprising (i) a pulse splitter operable
to receive the pulse from the source of radiation and to split the
pulse into multiple pulse portions, the pulse splitter comprising a
pulse-splitting surface; (ii) pulse-delaying optical paths situated
relative to the pulse splitter so as to receive the pulse portions
and cause the pulse portions to propagate along the pulse-delaying
optical paths and become relatively delayed with respect to each
other; (iii) a pulse recombiner operable to receive the relatively
delayed pulse portions from the pulse-delaying optical paths and to
direct the relatively delayed pulse portions along a direction so
as to form a pulse-width-extended pulse, the pulse recombiner
comprising a pulse-directing surface; and (iv) a relay optical
system situated so that the pulse-splitting surface is imaged onto
the pulse-directing surface; (c) an irradiation optical system
situated so as to receive the pulse-width-extended pulse and
illuminate the mask with the pulse-width-extended pulse; and (d) a
projection optical system operable to receive radiation, from the
pulse-width-extended pulse, transmitted through the mask and to
project the received radiation so as to form an image of the mask
pattern on the sensitized substrate.
2. The projection-exposure apparatus of claim 1, wherein the light
pulse has a pulse width that is a time during which the light pulse
has an intensity greater than one-half a maximum value of
intensity, a propagation speed, and a pulse length that is a
product of the pulse width and the propagation speed, and wherein
the pulse-delaying optical paths are operable to delay the pulse
portions, propagating along the pulse-delaying optical paths, with
respect to each other by a time greater than or equal to the
pulse-width.
3. The projection-exposure apparatus of claim 1, wherein the relay
optical system is situated to provide a magnification of either +1
or -1.
4. The projection-exposure apparatus of claim 3, wherein the pulse
splitter and the pulse recombiner comprise a beamsplitter, and the
pulse-delaying optical paths comprise a pulse-circulating optical
path along which the beamsplitter directs the pulse portions for
propagation around the pulse-circulating optical path in which the
pulse portions become relatively delayed, and from which
pulse-circulating optical path the beam splitter receives the
relatively delayed pulse portions for direction to the irradiation
optical system.
5. The projection-exposure apparatus of claim 4, wherein the
irradiation optical system comprises an optical member situated so
as to receive the pulse-width extended pulse and form a plurality
of light sources so as to direct radiation from the light sources
to the mask.
6. The projection-exposure apparatus of claim 1, wherein the pulse
splitter and the pulse recombiner comprise a beamsplitter, and the
pulse-delaying optical paths comprise a pulse-circulating optical
path along which the beamsplitter directs the pulse portions for
propagation around the pulse-circulating optical path in which the
pulse portions become relatively delayed, and from which
pulse-circulating optical path the beam splitter receives the
relatively delayed pulse portions for direction to the irradiation
optical system.
7. The projection-exposure apparatus of claim 1, wherein the
irradiation system comprises an optical member situated so as to
receive the pulse-width-extended pulse and form a plurality of
secondary sources, and a condenser operable to direct radiation
from the secondary sources onto the mask.
8. The projection-exposure apparatus of claim 7, wherein the pulse
of radiation has a pulse width that is a time during which the
light pulse has an intensity greater than one-half a maximum value
of intensity, a propagation speed, and a pulse length that is a
product of the pulse width and the propagation speed, and wherein
the pulse-delaying optical paths are operable to delay the pulse
portions, propagating along the pulse-delaying optical paths, with
respect to each other by a time greater than or equal to the
pulse-width.
9. The projection-exposure apparatus of claim 7, wherein the relay
optical system is situated to provide a magnification of either +1
or -1.
10. The projection-exposure apparatus of claim 9, wherein the pulse
splitter and the pulse recombiner comprise a beamsplitter, and the
pulse-delaying optical paths comprise a pulse-circulating optical
path along which the beamsplitter directs the pulse portions for
propagation around the pulse-circulating optical path in which the
pulse portions become relatively delayed, and from which
pulse-circulating optical path the beam splitter receives the
relatively delayed pulse portions for direction to the irradiation
optical system.
11. The projection-exposure apparatus of claim 10, wherein the
optical member comprises a fly-eye lens presenting a surface for
receiving the pulse-width-extended pulse, the surface of the
fly-eye lens being conjugate with the pulse-directing surface.
12. In a method of manufacturing a semiconductor device in which a
pattern is formed on a substrate by irradiating a pattern-defining
mask with radiation pulses from a radiation source, and forming an
image of the mask pattern on the substrate, an improvement
comprising the steps: (a) before the radiation pulses irradiate the
mask, directing the radiation pulses into a pulse-width extending
optical system, the radiation pulses each having a pulse width and
a peak radiation power; (b) passing the radiation pulses through
the pulse-width extending optical system so as to produce
pulse-width-extended radiation pulses each having a peak radiation
power that is less than the peak radiation power of the radiation
pulses from the radiation source; (c) directing the
pulse-width-extended radiation pulses to the mask; (d) irradiating
the substrate with the pulse-width-extended radiation pulses from
the mask; and (e) wherein step (b) comprises passing the radiation
pulses through a pulse-width extending optical system comprising
(i) a pulse splitter operable to receive the radiation pulses from
the radiation source and to split each of the radiation pulses into
multiple pulse portions, the pulse splitter comprising a
pulse-splitting surface; (ii) pulse-delaying optical paths situated
relative to the pulse splitter so as to receive the pulse portions
and cause the pulse portions to propagate along the pulse-delaying
optical paths and become relatively delayed with respect to each
other; (iii) a pulse recombiner operable to receive the relatively
delayed pulse portions from pulse-delaying optical paths and to
combine and direct the relatively delayed pulse portions along a
direction so as to form pulse-width-extended pulses, the pulse
recombiner comprising a pulse-directing surface; and (iv) a relay
optical system that images the pulse-splitting surface onto the
pulse-directing surface.
13. The method of claim 12, wherein step (c) further comprises
forming a plurality of light sources by using an optical member
that receives the pulse-width-extended pulses before the
pulse-width-extended pulses are directed to the mask.
14. The method of claim 13, wherein in step (c), the optical member
comprises a fly-eye lens.
15. The method of claim 12, wherein, in step (b), the pulse
splitter and the pulse recombiner of the pulse-width extending
optical system through which the radiation pulses are passed
comprise a beamsplitter, and the pulse-delaying optical paths
comprise a pulse-circulating optical path along which the
beamsplitter directs the pulse portions for propagation around the
pulse-circulating optical path in which the pulse portions become
relatively delayed, and from which pulse-circulating optical path
the beam splitter receives the relatively delayed pulse portions
for direction to the mask.
16. The method of claim 12, wherein: each of the radiation pulses
has a pulse width that is a time during which the radiation pulse
has an intensity greater than one-half a maximum value of
intensity, a propagation speed, and a pulse length that is a
product of the pulse width and the propagation speed, and in step
(b), the radiation pulses are passed through pulse-delaying optical
paths that differ from each other by at least the pulse length such
that the pulse portions propagating along the pulse-delaying
optical paths are delayed with respect to each other.
17. The method of claim 12, wherein, in step (c), the relay optical
system is situated to provide a magnification of either +1 or
-1.
18. A projection-exposure apparatus for projecting a pattern formed
on a mask onto a sensitized substrate, the apparatus comprising:
(a) a pulse-width extending optical system that includes a
pulse-splitting surface that splits a light pulse received from a
light source into multiple light-pulse portions, a pulse-delaying
optical path that introduces respective delays to the light-pulse
portions, a pulse-directing surface that recombines the delayed
light-pulse portions to form a pulse-width-extended light pulse
having a power, and a relay optical system that images the
pulse-splitting surface onto the pulse-directing surface; and (b)
an exposure optical system situated and configured to receive the
pulse-width-extended light pulse from the pulse-width extending
optical system, direct the pulse-width-extended light pulse to the
mask, and transfer the pattern from the mask to the sensitized
substrate; and wherein (c) the delays of the multiple light-pulse
portions are selected so that the power of the pulse-width-extended
light pulse is reduced to decrease degradation of the exposure
optical system.
19. The projection-exposure apparatus of claim 18, wherein the
pulse-width-extending optical system comprises a beamsplitter
situated to receive the light pulse from the light source and
configured to split the light pulse into the multiple light-pulse
portions, direct the multiple light-pulse portions along an optical
delay path, and recombine the delayed light-pulse portions, wherein
the beamsplitter has a reflectivity R such that
29.3%<R<50%.
20. The projection-exposure apparatus of claim 19, wherein: the
beamsplitter includes a beamsplitting surface that splits the light
pulse into the multiple light-pulse portions; and the pulse-width
extending optical system comprises a re-imaging system situated and
configured to image the beamsplitting surface onto the
beamsplitting surface at a pre-determined magnification.
21. The projection-exposure apparatus of claim 18, wherein the
pulse-width-extending optical system comprises a beamsplitter
situated to receive the light pulse from the light source and
configured to split the light pulse into the multiple light-pulse
portions, and an optical delay path situated and configured to
introduce the respective delays to each light-pulse portion and to
direct the delayed light-pulse portions to the beamsplitter, the
beamsplitter having transmissivity T such that
29.3%<T<50%.
22. The projection-exposure apparatus of claim 21, wherein: the
beamsplitter comprises a beamsplitting surface situated and
configured to split the light pulse into the multiple light-pulse
portions; and the optical delay path comprises an imaging optical
system situated and configured to image the beamsplitting surface
onto the beamsplitting surface at a predetermined
magnification.
23. The projection-exposure apparatus of claim 18, wherein the
pulse-width-extending optical system comprises a polarizing beam
splitter situated to receive the light pulse from the light source
and configured to split the light pulse into the multiple
light-pulse portions.
24. The projection-exposure apparatus of claim 23, wherein the
pulse-width extending system further comprises an optical delay
path situated and configured to direct the multiple light-pulse
portions from the polarizing beamsplitter back to the polarizing
beamsplitter, and a quarter-wave retarder situated in the optical
delay path.
25. The projection-exposure apparatus of claim 24, wherein: the
polarizing beamsplitter comprises a beamsplitting surface that
splits the light pulse from the light source into the multiple
light-pulse portions; and the optical delay path comprises an
imaging optical system situated and configured to image the
beamsplitting surface onto the beamsplitting surface at a
predetermined magnification.
26. The projection-exposure apparatus of claim 18, wherein the
exposure optical system comprises: an illumination system situated
and configured to direct the pulse-width-extended light pulse to
the mask; and a projection optical system situated and configured
to project an image of the pattern from the mask onto the
sensitized substrate.
27. The projection-exposure apparatus of claim 26, wherein the
illumination system further comprises: a fly-eye lens situated and
configured to form multiple light sources from the
pulse-width-extended light pulse; and a condensing optical system
situated and configured to direct light from the multiple light
sources to the mask.
28. A method for exposing a sensitized substrate to a pattern
defined by a mask, comprising: (a) producing a light pulse; (b)
directing the light pulse through a pulse-width extending optical
system to split the light pulse into multiple light-pulse portions
and delay each light-pulse portion to produce a
pulse-width-extended light pulse, the pulse-width extending optical
system comprising (i) a pulse splitter operable to receive the
light pulse from the radiation source and to split the light pulse
into multiple pulse portions, the pulse splitter comprising a
pulse-splitting surface; (ii) pulse-delaying optical paths situated
relative to the pulse splitter so as to receive the pulse portions
and cause the pulse portions to propagate along the pulse-delaying
optical paths and become relatively delayed with respect to each
other; (iii) a pulse recombiner operable to receive the relatively
delayed pulse portions from the pulse-delaying optical paths and to
combine and direct the relatively delayed pulse portions along a
direction so as to form pulse-width-extended pulses, the pulse
recombiner comprising a pulse-directing surface; and (iv) a relay
optical system that images the pulse-splitting surface onto the
pulse-directing surface; (c) directing the pulse-width-extended
light pulse to an exposure optical system; (d) irradiating the
pattern defined by the mask with the pulse-width-extended light
pulse from the exposure optical system; (e) imaging the pattern
onto a sensitized substrate, and (f) delaying each light-pulse
portion in the pulse-width extending optical system sufficiently to
reduce deterioration in optical properties of the exposure optical
system.
29. The method of claim 28, wherein the pulse-width extending
optical system of step (b) comprises a beamsplitter and an optical
delay path, the beamsplitter situated and configured to split the
light pulse into the multiple light-pulse portions and to direct
the light-pulse portions along the optical delay path from the
beamsplitter back to the beamsplitter, the beamsplitter having a
reflectivity R such that 29.3%<R<50%.
30. The method of claim 29, wherein: the beamsplitter has a
beamsplitting surface that splits the light pulse into the multiple
light-pulse portions; and the optical delay path includes an
imaging optical system situated to receive the multiple light-pulse
portions and configured to image the beamsplitting surface onto the
beamsplitting surface at a predetermined magnification.
31. The method of claim 28, wherein the pulse-width extending
optical system includes a beamsplitter situated and configured to
split the light pulse into the multiple light-pulse portions and an
optical delay path situated and configured to direct the multiple
light-pulse portions from the beamsplitter back to the
beamsplitter, the beamsplitter having a transmissivity T such that
29.3%<T<50%.
32. The method of claim 31, wherein: the beamsplitter comprises a
beamsplitting surface that splits the light pulse into the multiple
light-pulse portions; and the optical delay path includes an
imaging optical system situated and configured to image the
beamsplitting surface onto the beamsplitting surface at a specified
magnification.
33. The method of claim 28, wherein the pulse-width extending
optical system includes a polarizing beam splitter situated and
configured so as to split the light pulse into the multiple
light-pulse portions.
34. The method of claim 33, wherein the pulse-width extending
optical system further comprises an optical delay path situated and
configured to direct the light-pulse portions from the polarizing
beamsplitter back to the polarizing beam splitter, the optical
delay path including a quarter-wave retarder.
35. The method of claim 34, wherein: the polarizing beam splitter
includes a beamsplitting surface that splits the light pulse into
the multiple light-pulse portions; and the optical delay path
includes an imaging optical system situated and configured to image
the beamsplitting surface onto the beamsplitting surface at a
specified magnification.
36. An exposure apparatus for transferring patterns from a mask to
a substrate, comprising: (a) a light source that generates a light
pulse; (b) a secondary light-source system situated and configured
to receive the light pulse, form multiple secondary light sources,
and direct light from the secondary light sources to the mask; (c)
a projection optical system situated and configured to project an
image of the pattern from the mask onto a sensitized substrate; and
(d) a pulse-width extending optical system situated to receive the
light pulse and configured to direct the light pulse to the
secondary light-source system and to extend a pulse length of the
light pulse, the pulse-width extending optical system comprising
(i) a pulse splitter operable to receive the light pulse from the
radiation source and to split the light pulse into multiple pulse
portions, the pulse splitter comprising a pulse-splitting surface;
(ii) pulse-delaying optical paths situated relative to the pulse
splitter so as to receive the pulse portions and cause the pulse
portions to propagate along the pulse-delaying optical paths and
become relatively delayed with respect to each other; (iii) a pulse
recombiner operable to receive the relatively delayed pulse
portions from the pulse-delaying optical paths and to combine and
direct the relatively delayed pulse portions along a direction so
as to form pulse-width-extended pulses, the pulse recombiner
comprising a pulse-directing surface; and (iv) a relay optical
system that images the pulse-splitting surface onto the
pulse-directing surface.
37. The exposure apparatus of claim 36, further comprising a
condenser optical system situated to receive the light from the
secondary light-source system and configured to direct the light
from the secondary light sources to the mask.
38. The exposure apparatus of claim 36, wherein the secondary
light-source system includes a fly-eye lens.
39. The exposure apparatus of claim 36, wherein the pulse-width
extending optical system comprises: a beamsplitter that includes a
beamsplitting surface that splits the light pulse into multiple
light-pulse portions; and a light guide system that directs the
multiple light-pulse portions along an optical delay path to the
beamsplitter, the light guide system including an imaging system
situated and configured to image the beamsplitting surface onto the
beamsplitting surface at a predetermined magnification.
40. The exposure apparatus of claim 36, wherein the pulse-width
extending optical system comprises a beamsplitter having a
reflectance R such that 29.3%<R<50%.
41. The exposure apparatus of claim 36, wherein the pulse-width
extending optical system comprises a beamsplitter having a
transmittance T such that 29.3%<T<50%.
42. The exposure apparatus of claim 36, wherein the pulse-width
extending optical system comprises a polarizing beam splitter.
43. An exposure method for transferring a pattern from a mask to a
substrate, comprising: (a) providing a light pulse having a pulse
length; (b) extending the pulse length of the light pulse with a
pulse-width extending optical system comprising (i) a pulse
splitter operable to receive the light pulse from the radiation
source and to split the light pulse into multiple pulse portions,
the pulse splitter comprising a pulse-splitting surface; (ii)
pulse-delaying optical paths situated relative to the pulse
splitter so as to receive the pulse portions and cause the pulse
portions to propagate along the pulse-delaying optical paths and
become relatively delayed with respect to each other; (iii) a pulse
recombiner operable to receive the relatively delayed pulse
portions from the pulse-delaying optical paths and to combine and
direct the relatively delayed pulse portions along a direction so
as to form pulse-width-extended pulses, the pulse recombiner
comprising a pulse-directing surface; and (iv) a relay optical
system that images the pulse-splitting surface onto the
pulse-directing surface; (c) forming multiple secondary light
sources from the light pulse after step (b); and (d) exposing the
substrate to the pattern from the mask using the multiple secondary
light sources.
44. The method of claim 43, wherein step (d) comprises: irradiating
the pattern of the mask with light from the multiple secondary
sources; and projecting an image of the pattern onto the
substrate.
45. The method of claim 43, wherein the multiple secondary sources
are formed with a secondary light-source system that includes a
fly-eye lens.
46. The method of claim 43, wherein step (b) is performed with a
pulse-width extending optical system, the pulse-width extending
optical system including: a beamsplitter including a beam splitting
surface that splits the light pulse into multiple light-pulse
portions; and a light-guide system that directs the multiple
light-pulse portions along an optical delay path to the beam
splitter, the light-guide system including an imaging system
situated and configured to image the beamsplitting surface onto the
beamsplitting surface at a predetermined magnification.
47. The method of claim 43, wherein step (b) is performed by using
a pulse-width extending optical system that includes a beamsplitter
having a reflectance R such that 29.3%<R<50%.
48. The method of claim 43, wherein step (b) is performed by using
a pulse-width extending optical system that includes a beamsplitter
having a transmittance T such that 29.3%<T<50%.
49. The method of claim 43, wherein step (b) is performed by using
a pulse-width extending optical system that includes a polarizing
beamsplitter.
50. An exposure apparatus comprising: (a) a radiation source
configured to emit a pulse of radiation; (b) an illumination
optical system that is situated to receive the pulse of radiation
and configured to direct the pulse of radiation to a mask; (c) a
projection optical system situated and configured to project an
image of the mask onto a substrate; and (d) a pulse-width-extending
optical system situated to receive the pulse of radiation and
configured to produce at least two pulse portions having relative
delays, the pulse-width-extending optical system including (i) a
pulse splitter having either a transmittance T such that
29.3%<T<50% or a reflectance R such that 29.3%<R<50%,
and (ii) a relay optical system situated to receive at least one
pulse portion and configured to direct the received pulse portion
from the pulse splitter back to the pulse splitter and to form an
image of the pulse splitter on the pulse splitter.
51. The exposure apparatus of claim 50, wherein the relay optical
system is configured to produce a magnification of +1 or -1.
52. The exposure apparatus of claim 50, wherein the illumination
optical system includes an optical integrator situated to receive
the pulse portions from the pulse-width-extending optical system
and configured to form multiple light sources, and a condenser
situated and configured to direct radiation from the multiple light
sources to the mask.
53. The exposure apparatus of claim 52, wherein the optical
integrator includes a multi-lens array.
54. The exposure apparatus of claim 53, wherein the multi-lens
array includes a fly-eye lens.
55. The exposure apparatus of claim 50, further comprising a
radiation relay optical system situated and configured to direct
the pulse of radiation to the pulse-width extending-optical
system.
56. A method of manufacturing a semiconductor device, comprising:
(a) providing a radiation source that emits a pulse of radiation;
(b) providing a pulse-width extending-optical system that divides
the pulse of radiation into two or more pulse portions, wherein the
pulse portions are relatively delayed with respect to each other,
the pulse-width-extending optical system including (i) a
pulse-separating surface situated to receive the pulse of radiation
and having either a transmittance T such that 29.3%<T<50% or
a reflectance R such that 29.3%<R<50%, and (ii) a relay
optical system situated to direct a pulse portion from the
pulse-separating surface back to the pulse-separating surface and
that images the pulse-separating surface onto the pulse-separating
surface; (c) directing the pulse portions to a mask; and (d)
projecting an image of the mask onto a substrate.
57. The method of claim 56, further comprising configuring the
relay optical system to produce a magnification of either +1 or
-1.
58. The method of claim 56, further comprising: situating an
optical integrator to receive the pulse portions; and passing the
pulse portions through a condenser optical system to guide the
pulse portions from the optical integrator to the mask.
59. The method of claim 56, wherein the optical integrator includes
a multi-lens array.
60. The method of claim 59, wherein the multi-lens array includes a
fly-eye lens.
61. The method of claim 56, further comprising directing the pulse
of radiation to the pulse-width-extending optical system with a
radiation relay optical system.
62. An exposure apparatus comprising: a light source unit supplying
a light pulse to transfer a pattern formed on a mask onto a
photosensitive substrate; an illumination optical system arranged
in an optical path between said mask and said light source unit,
said illumination optical system comprising a uniform illumination
unit disposed in an optical path within said illumination optical
system so as to perform uniform illumination with respect to the
mask; and a pulse-width extending optical system arranged in an
optical path between said light source and said uniform
illumination unit, which extends a duration of the light pulse
supplied from said light source while decreasing a peak power of
the light pulse, said pluse-width extending optical system
including an optical surface having a multi-layer coating dividing
the light pulse into at least two light pulses.
63. An exposure apparatus according to claim 62, wherein said
pulse-width extending optical system comprises a beam dividing
member disposed to divide the light pulse into multiple pulse
portions, and a beam deflecting system disposed to direct at least
one of the pulse portions to said beam dividing member, wherein the
beam dividing member includes the optical surface having the
multi-layer coating.
64. An exposure apparatus according to claim 63, wherein: said beam
dividing member comprises a pulse-splitting surface and a
pulse-combining surface, and said pulse-width extending optical
system further comprises a relay optical system that images the
pulse-splitting surface onto the pulse-combining surface.
65. An exposure apparatus according to claim 64, wherein said relay
optical system has a magnification of either +1 or -1.
66. An exposure apparatus according to claim 65, wherein said light
source unit comprises an excimer laser source.
67. An exposure apparatus according to claim 64, wherein the
pulse-splitting surface and the pulse-combining surface are the
same surface.
68. An exposure apparatus according to claim 64, wherein said light
source unit comprises an excimer laser source.
69. An exposure apparatus according to claim 63, wherein said beam
dividing member comprises a polarizing beam splitter.
70. An exposure apparatus according to claim 69, wherein said light
source unit comprises an excimer laser source.
71. An exposure apparatus according to claim 63, wherein said light
source unit comprises an excimer laser source.
72. An exposure apparatus according to claim 63, wherein said beam
deflecting system comprises a plurality of deflecting members
disposed to deflect said at least one of the pulse portions.
73. An exposure apparatus according to claim 63, wherein said
pulse-width extending optical system divides said light pulse into
a plurality of pulse portions to give predetermined differences
between optical path lengths through which said divided pulse
portions pass respectively, and said predetermined differences in
the optical path lengths is larger than a product of a time period
during which an emission intensity of the light pulse is larger
than one-half of a peak intensity of the light pulse in the
emission of said light pulse, and a velocity of light.
74. An exposure apparatus according to claim 62, wherein the
uniform illumination unit comprises a fly-eye lens.
75. An exposure apparatus comprising: a light source unit supplying
a light pulse to transfer a pattern formed on a mask onto a
photosensitive substrate; an illumination optical system arranged
in an optical path between said mask and said light source unit,
said illumination optical system comprising a uniform illumination
unit disposed in an optical path within said illumination optical
system so as to perform uniform illumination with respect to the
mask; and a pulse-width extending optical system arranged in an
optical path between said light source unit and said uniform
illumination unit, which extends a duration of the light pulse
supplied from said light source unit while decreasing a peak power
of the light pulse, said pulse-width extending optical system
including a polarizing beam splitter.
76. An exposure apparatus comprising: a light source unit supplying
a light pulse to transfer a pattern formed on a mask onto a
photosensitive substrate; an illumination optical system arranged
in an optical path between said mask and said light source unit,
said illumination optical system comprising a uniform illumination
unit disposed in an optical path within said illumination optical
system so as to perform uniform illumination with respect to the
mask; and a pulse-width extending optical system arranged in an
optical path between said light source unit and said uniform
illumination unit, which extends a duration of the light pulse
supplied from said light source unit while decreasing a peak power
of the light pulse, said pulse-width extending optical system
including a beam converting member that changes a polarized beam
condition.
77. An exposure apparatus according to claim 76, wherein said beam
converting member comprises a half wavelength plate.
78. An exposure apparatus according to claim 77, wherein said
pulse-width extending optical system divides said light pulse into
a plurality of pulse portions to give predetermined differences
between optical path lengths through which said divided pulse
portions pass respectively, and said predetermined differences in
the optical path lengths is larger than a product of a time period
during which an emission intensity of the light pulse is larger
than one-half of a peak intensity of the light pulse in the
emission of said light pulse, and a velocity of light.
79. An exposure apparatus comprising: a light source unit supplying
a light pulse with energy of at least 10 mw/cm.sup.2 and a pulse
width .delta.[sec] to transfer a pattern formed on a mask onto a
photosensitive substrate; an illumination optical system arranged
in an optical path between said mask and said light source unit,
said illumination optical system comprising a uniform illumination
unit disposed in an optical path within said illumination optical
system so as to perform uniform illumination with respect to the
mask; and a pulse-width extending optical system arranged in an
optical path between said light source unit and said uniform
illumination unit, which extends a duration of the light pulse
supplied form said light source unit while decreasing a peak power
of the light pulse, said pulse-width extending optical system
divides the light pulse into multiple pulse portions and provides a
pulse length of at least 3.times.10.sup.8.times..delta.[m] with
respect to said divided pulse portions.
80. A method of manufacturing a semiconductor device comprising the
steps of: supplying a light pulse; producing pulse-width-extended
radiation pulses which extend a duration of the light pulse while
decreasing a peak power of the light pulse; performing a uniform
illumination with respect to a mask by using a uniform illumination
unit; and transferring a pattern formed on the mask onto a
photosensitive substrate; wherein said producing step includes a
step of using an optical surface having a multi-layer coating
dividing the light pulse into at least two light pulses.
81. A method according to claim 80, wherein said producing step
comprises the steps of: dividing the light pulse into multiple
pulse portions by using a beam dividing member which includes the
optical surface having the multi-layer coating; and directing at
least one of the pulse portions to said beam dividing member by a
deflecting system.
82. A method according to claim 81, wherein said pulse portion
directing step comprises the step of imaging a pulse-splitting
surface of said beam dividing member onto a pulse-combining surface
of said beam dividing member by using a relay optical system.
83. A method according to claim 82, wherein said relay optical
system has a magnification of either +1 or -1.
84. A method according to claim 83, wherein said supplying step
comprises the step of using an excimer laser.
85. A method according to claim 82, wherein the pulse-splitting
surface and the pulse-combining surface are the same surface.
86. A method according to claim 82, wherein said supplying step
comprises the step of using an excimer laser.
87. A method according to claim 81, wherein said beam dividing
member comprises a polarizing beam splitter.
88. A method according to claim 87, wherein said supplying step
comprises the step of using an excimer laser.
89. A method according to claim 81, wherein said supplying step
comprises the step of using an excimer laser.
90. A method according to claim 81, wherein said producing step
comprises a step of dividing said light pulse into a plurality of
pulse portions and giving predetermined differences between optical
path lengths through which said divided pulse portions pass
respectively, wherein said predetermined differences in the optical
path lengths is larger than a product of a time period during which
an emission intensity of the light pulse is larger than one-half of
a peak intensity of the light pulse in the emission of said light
pulse, and a velocity of light.
91. A method according to claim 80, wherein said supplying step
comprises the step of using an excimer laser.
92. A method of manufacturing a semiconductor device comprising the
steps of: supplying a light pulse; producing pulse-width-extended
radiation pulses which extend a duration of the light pulse while
decreasing a peak power of the light pulse; performing a uniform
illumination with respect to a mask by using a uniform illumination
unit; and transferring a pattern formed on the mask onto a
photosensitive substrate; wherein said producing step includes the
step of using a polarizing beam splitter.
93. A method according to claim 92, wherein said producing step
comprises a step of dividing said light pulse into a plurality of
pulse portions and giving predetermined differences between optical
path lengths through which said divided pulse portions pass
respectively, wherein said predetermined differences in the optical
path lengths is larger than a product of a time period during which
an emission intensity of the light pulse is larger than one-half of
a peak intensity of the light pulse in the emission of said light
pulse, and a velocity of light.
94. A method of manufacturing a semiconductor device comprising the
steps of: supplying a light pulse; producing pulse-width-extended
radiation pulses which extend a duration of the light pulse while
decreasing a peak power of the light pulse; performing a uniform
illumination with respect to a mask by using a uniform illumination
unit; and transferring a pattern formed on the mask onto a
photosensitive substrate; wherein said producing step includes the
step of using a beam converting member that changes a polarized
beam condition.
95. A method according to claim 94, wherein said beam converting
member includes a half wavelength plate.
96. A method of manufacturing a semiconductor device comprising the
steps of: supplying a light pulse with energy of at least 10
mw/cm.sup.2 and a pulse width .delta.[sec]; producing
pulse-width-extended radiation pulses which extend a duration of
the light pulse while decreasing a peak power of the light pulse;
performing a uniform illumination with respect to a mask by using a
uniform illumination unit; and transferring a pattern formed on the
mask onto a photosensitive substrate; wherein said producing step
comprises the steps of: dividing the light pulse into multiple
pulse portions; and providing a pluse length of at least
3.times.10.sup.8.times..delta.[m] with respect to said divided
pulse portions.
97. An exposure apparatus comprising: an excimer laser source
supplying a light pulse to transfer a pattern formed on a mask onto
a photosensitive substrate; an illumination optical system arranged
in an optical path between said mask and said excimer laser source;
and a pulse-width extending optical system arranged in an optical
path between said excimer laser source and said illumination
optical system, which extends a duration of the light pulse
supplied from said excimer laser source while decreasing a peak
power of the light pulse, wherein said pulse-width extending
optical system includes a light split member having a light split
surface with a reflectance R such that 29.3%<R<50% or a
transmittance T such that 29.3%<T<50%.
98. An exposure apparatus according to claim 97, further comprising
a projection optical system disposed in an optical path between the
mask and the photosensitive substrate so as to project an image of
a pattern formed on the mask onto the photosensitive substrate.
99. A method of manufacturing a semiconductor device comprising the
steps of: supplying a light pulse by using an excimer laser source;
producing pulse-width-extended radiation pulses which extend a
duration of the light pulse while decreasing a peak power of the
light pulse, said producing step including a step of using a light
split member having a light split surface with a reflectance R such
that 29.3%<R<50% or a transmittance T such that
29.3%<T<50%; directing the pulse-width-extended radiation
pulses to a mask; and transferring a pattern formed on the mask
onto a photosensitive substrate.
100. An exposure apparatus comprising: a light source supplying a
light pulse; an exposure optical system arranged in an optical path
between said light source and a photosensitive substrate so as to
transfer a pattern formed on a mask onto the photosensitive
substrate, said exposure optical system comprising a plurality of
refractive optical elements which are made from at least one of
fused silica and fluorite; and a pulse-width extending optical
system arranged in an optical path between said light source and
said exposure optical system, which extends a duration of the light
pulse supplied from said light source while decreasing a peak power
of the light pulse so as to maintain an optical performance with
respect to said plurality of refractive optical elements.
101. An exposure apparatus according to claim 100, wherein said
light source includes an excimer laser source.
102. An exposure apparatus according to claim 100, wherein said
exposure optical system comprises an illumination optical system
disposed in an optical path between said light source and the mask
so as to illuminate the mask, and a projection optical system
disposed in an optical path between the mask and the photosensitive
substrate.
103. An exposure apparatus according to claim 100, wherein each of
said plurality of refractive optical elements includes an
anti-reflection coating.
104. An exposure apparatus according to claim 100, wherein said
pulse-width extending optical system includes a light split member
having a light split surface with a reflectance R such that
29.3%<R<50% or a transmittance T such that
29.3%<T<50%.
105. An exposure apparatus according to claim 100, further
comprising a projection optical system disposed in an optical path
between the mask and the photosensitive substrate so as to project
an image of a pattern formed on the mask onto the photosensitive
substrate.
106. A method of manufacturing a semiconductor device comprising
the steps of: supplying a light pulse; transferring a pattern
formed on a mask onto a photosensitive substrate by using the light
pulse passing through an exposure optical system comprising a
plurality of refractive optical elements which are made from at
least one of fused silica and fluorite; and producing
pulse-width-extended radiation pulses which extend a duration of
the light pulse while decreasing a peak power of the light pulse so
as to maintain an optical performance with respect to said
plurality of refractive optical elements before said transferring
step.
107. A method according to claim 106, wherein said light pulse is
supplied from an excimer laser source.
108. A method according to claim 106, wherein each of said
plurality of refractive optical elements includes an
anti-reflection coating.
109. A method according to claim 106, wherein said producing step
includes a step of using a light split member having a light split
surface with a reflectance R such that 29.3%<R<50% or a
transmittance T such that 29.3%<T<50%.
Description
FIELD OF THE INVENTION
This invention pertains to pulse-width extending optical systems
and a projection-exposure apparatus using such optical systems.
More specifically, the invention pertains to optical systems that
extend the duration of light pulses emitted by a pulsed laser, and
to projection-exposure apparatus using such optical systems. The
invention also pertains to methods for manufacturing semiconductor
devices using such projection-exposure apparatus.
BACKGROUND OF THE INVENTION
FIG. 16 is a block diagram of a conventional microlithography
projection-exposure apparatus comprising a pulsed laser. This
projection-exposure apparatus comprises a laser 1 that emits light
pulses. A beam-shaping optical system 2 shapes the cross section of
the beam. The beam then enters a fly-eye lens 3. The fly-eye lens 3
divides the incident laser beam into multiple secondary light
sources, one such secondary light source being formed at the rear
focal point of the fly-eye lens 3. An aperture 4 limits the beam,
and a condenser lens 5 uniformly illuminates a mask 6 with the
beam. Typically the mask 6 contains a high-resolution pattern with
extremely small features, e.g., patterns for semiconductor
integrated circuits. A projection optical system 7 projects the
pattern of the mask 6 on a wafer 8. The projected pattern may be
either demagnified (reduced) or magnified (enlarged).
The resolution of the pattern of the mask 6 as projected on the
wafer 8 depends on the wavelength of the light from the laser 1.
The laser 1 emits light having as short a wavelength as possible in
order to form high-resolution patterns on the wafer 8.
Many lasers emitting short wavelengths of light emit pulses of
light. The peak optical powers and intensities of pulsed lasers are
very much larger than their average optical powers. (Optical power
is defined as optical energy per unit time; optical intensity is
optical power per unit area.) For example, for an ArF excimer laser
which emits light at a wavelength of 193 nm and which has a beam
cross section of 20 mm by 5 mm, typical peak pulse intensities
during pulses are on the order of 10 MW/cm.sup.2.
Short-wavelength radiation tends to cause changes in optical
materials. These changes include increased absorption by the
materials and radiation-induced changes in refractive index. These
changes are frequently irreversible. In addition, such changes are
more readily produced by high power and high-intensity radiation in
comparison with radiation of similar average power but lesser peak
values. For this reason, systems using short-wavelength lasers
often suffer from radiation-induced changes to their optical
elements.
Conventional projection-exposure apparatus using short-wavelength
lasers also exhibit astigmatism caused by variations in the
refractive indices of the lens material of the projection optical
system. Such astigmatism significantly degrades the resolution of
the projection optical systems.
For example, the ArF excimer laser (emission wavelength of 193 nm)
is a suitable short-wavelength laser. Only a few refractive optical
materials are appropriate for use in optical systems with this
short wavelength. The most commonly used materials are synthetic
fused quartz and fluorite. Both of these materials show a gradual
decline in transmissivity when irradiated by light of intensities
greater than certain threshold intensities. In order to prevent a
decline in transmissivity, the optical systems of
projection-exposure apparatus frequently enlarge the diameter of
the light beam so as to reduce the intensity of optical pulses on
the lenses.
SUMMARY OF THE INVENTION
This invention provides pulse-width extending optical systems,
projection-exposure apparatus comprising such systems, and
manufacturing methods using such projection-exposure apparatus. The
pulse-width extending systems lower laser peak powers in the
optical system without reducing average laser power. For
convenience, optical pulse width is defined as the time during
which an optical pulse has an intensity greater than one-half of
the maximum value of the intensity. It is also convenient to define
an optical pulse length as the distance traveled by an optical
pulse in a time equal to its pulse width.
In a preferred embodiment of a pulse-width extending optical system
according to the invention, a beamsplitter splits an incident laser
pulse into two or more pulses. The pulses propagate along optical
paths such that they are delayed with respect to each other. A
beamsplitter then receives the delayed pulses and directs them
along a common output optical path. In the example embodiments, one
or more beamsplitters split and combine the pulses.
Because the split pulses are delayed, the peak power at the output
is reduced because optical pulse energy from the original pulse
arrives over a time period that is longer than the original pulse
duration. The delay among the pulses is set by causing the split
pulses to travel different optical paths. To effectively reduce the
laser power, the optical path differences are preferably greater
than the pulse length.
Pulse-width extending optical systems according to the invention
also preferably comprise relay systems operable to ensure that the
delayed pulses maintain appropriate beam cross-sections and do not
become large because of the natural divergence of light beams. The
relay systems provide an additional benefit. If a relay system
inverts a beam image, then beam uniformity is improved because the
relay system overlaps a beam and an inverted image.
According to another aspect of the present invention,
projection-exposure apparatus are provided comprising pulse-width
extending optical systems as summarized above. In such a
projection-exposure apparatus, the pulse-width extended radiation
is directed to a multi-source image mechanism to form multiple
images of the pulse-width extended radiation. A condenser then
illuminates the mask substantially uniformly using the multi-source
images. This projection-exposure apparatus can be advantageously
used for manufacturing semiconductor devices.
The foregoing and other objects, features, and advantages of the
invention will become more apparent from the following detailed
description of the example embodiments which proceeds with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DIAGRAMS
FIG. 1 is an optical block diagram of a pulse-width extending
optical system according to a first example embodiment of the
invention.
FIG. 2 is a plot of laser power as a function of time for the laser
of the pulse-width extending optical system of FIG. 1.
FIG. 3 is a plot of laser power as a function of time for multiple
split laser pulses without temporal overlap.
FIG. 4 is a plot of power as a function of time for multiple split
pulses in which the optical path delay is one-half the laser
pulse-width.
FIG. 5 is an optical block diagram of a pulse-width extending
optical system according to a second example embodiment of the
invention.
FIG. 6 is an optical block diagram of a pulse-width extending
optical system according to a third example embodiment of the
invention.
FIG. 7 is an optical block diagram of a pulse-width extending
optical system according to a fourth example embodiment of the
invention.
FIG. 8 is an optical block diagram of a pulse-width extending
optical system according to a fifth example embodiment of the
invention.
FIG. 9 is an optical block diagram of a pulse-width extending
optical system according to a sixth example embodiment of the
invention.
FIG. 10 is a plot of the optical beam cross-section for the delayed
optical pulses of the optical system of the second example
embodiment.
FIG. 11 is a plot of the optical beam cross-section for an
asymmetric optical pulse from an excimer laser according to the
sixth example embodiment.
FIG. 12 is a plot of the beam cross-section of alternating delayed
optical pulses showing that even-numbered pulses are inverted with
respect to odd-number pulses in a delaying optical system with a
relay optical system such as that of the sixth example
embodiment.
FIG. 13 is an optical block diagram of a pulse-width extending
optical system according to a seventh example embodiment of the
invention.
FIG. 14 is an optical block diagram of a pulse-width extending
optical system according to an eighth example embodiment of the
invention.
FIG. 15 is an optical block diagram of a projection-exposure
apparatus comprising a pulse-width extending optical system
according to a ninth example embodiment of the invention.
FIG. 16 is an optical block diagram of a prior-art
projection-exposure apparatus employing a pulsed laser.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The duration of a light pulse can be increased by splitting it into
multiple sub-pulses, causing the sub-pulses to propagate differing
distances thereby acquiring differing delays, and then directing
the sub-pulses back along a common axis. By increasing the duration
of the pulse, the peak pulse power decreases. If pulse splitting
and recombining introduces insignificant optical loss, then the
recombined pulses have approximately the same total energy as the
initial light pulse. Thus, the average optical power is unchanged,
but the peak power decreases. By decreasing the peak optical power,
optical-material degradation caused by high powers is reduced. This
is especially important for projection-exposure apparatus utilizing
pulsed lasers where optical-material degradation degrades the
performance of the projection optical system in such an
apparatus.
FIG. 1 is an optical block diagram of a pulse-width extending
optical system according to a first example embodiment. FIG. 2
shows a representative plot of the intensity, as a function of
time, of the laser pulses as emitted by the laser 1 of the FIG. 1
embodiment.
The pulse-width extending optical system of FIG. 1 comprises a
laser 1 that emits pulses of nearly linearly polarized light. The
polarization orientation of the pulses emitted by the laser 1 is
either perpendicular or parallel to the plane of the page of FIG.
1.
A light pulse emitted by the laser 1 strikes a dielectric
beamsplitter 10, which has a partially transmitting, dielectric
multi-layer coating. The dielectric beamsplitter 10 divides the
laser pulse into a first output pulse that is reflected by the
dielectric beamsplitter 10, and a first circulating pulse that is
transmitted through the dielectric beamsplitter 10. The first
output pulse from the dielectric beamsplitter 10 then strikes a
mirror 14 that directs the pulse to an output. The first
circulating pulse is transmitted by the dielectric beamsplitter 10
and is then reflected sequentially by mirrors 11, 12, 13 before
returning to the dielectric beamsplitter 10. The mirrors 11, 12, 13
and the dielectric beamsplitter form a circulating optical path
("circulator") for the circulating pulse.
The dielectric beamsplitter 10 then divides the returning pulse
into a second output pulse and a second circulating pulse. The
second output pulse is the portion of the first circulating pulse
that is transmitted through the dielectric beamsplitter 10. The
mirror 14 reflects the second output pulse to an output. The first
circulating pulse is partially reflected by the dielectric
beamsplitter 10, forming the second circulating pulse. The second
circulating pulse follows the same optical path as the first
circulating pulse. The mirrors 11, 12, 13 sequentially reflect the
second circulating pulse and return it to the dielectric
beamsplitter 10.
The dielectric beamsplitter 10 once again divides the returning
pulse into a third output pulse and a third circulating pulse. The
third output pulse follows the optical path of the second output
pulse and the first output pulse; the mirror 14 reflects this pulse
to an output. The third circulating pulse similarly follows the
optical path of the first and second circulating pulses. The
mirrors 11, 12, 13 reflect the third circulating pulse and redirect
it to the dielectric beamsplitter 10. The dielectric beamsplitter
10 once again divides the returning pulse in the same way as the
prior circulating pulses were divided.
In this way, pulses circulate along the circulating optical path,
from mirror 11 to mirror 12 to mirror 13 and back to the dielectric
beamsplitter 10. The dielectric beamsplitter 10 transmits a portion
of the circulating pulse so the successive circulating pulses
decrease in power. The output pulses are all directed along
substantially the same axis as the first output pulse. Therefore,
downstream optical systems at the output will redirect all output
pulses in same way. If the mirrors 11, 12, 13 and the dielectric
beamsplitter 10 are perfect (i.e., exhibit no loss of light energy
reflecting therefrom), then all of the light emitted by the laser 1
is eventually reflected to the output by the mirror 14.
The dielectric beamsplitter 10 thus splits a single incident laser
pulse into multiple pulses that travel differing distances. The
dielectric beamsplitter 10 also recombines the multiple pulses and
directs them in a single direction. The circulating optical path of
the circulator is formed by the disposition of the three mirrors
11, 12, 13 so as to provide a predetermined optical-path-length
difference or form a pulse-delay optical path. The mirrors 11, 12,
13 have the function of an optical system that provides an
optical-path-length difference or forms a pulse-delay optical
system.
If the pulse width of the pulses from the laser 1 is .delta. (sec),
then the pulse travels a distance ("pulse length") during a time
equal to the pulse width of 3.times.10.sup.8.times..delta.[m],
wherein it is assumed that the pulse travels in a medium having a
refractive index n=1. If the optical path length of the circulator
(i.e., the dielectric beamsplitter 10 and the mirrors 11, 12, 13)
is longer than the pulse length, then the pulses delivered to the
mirror 14 through the dielectric beamsplitter 10 will have no
temporal overlap. FIG. 3 shows the output-pulse power as a function
of time for a circulator in which the output pulses have no
temporal overlap.
For example, if the pulse width .delta. is 10 nsec, the pulse
length is 3 m. If the optical path length of the circulator is
greater than 3 m, then the recombined pulses have no temporal
overlap. FIG. 3 shows that the circulator extends the optical pulse
width and the optical power (energy per unit of time) is
significantly reduced.
Even if the optical path length of the circulator is less than or
equal to the pulse length, if each pulse is sufficiently delayed or
advanced with respect to other pulses, then the peak power of the
recombined beam is still smaller than the peak power of the
original laser pulse entering the circulator. The peak optical
power can be effectively reduced by setting the optical path
difference to half the pulse length. As shown in FIG. 2, the pulse
width is defined as the time during which the pulse has a power
greater than one-half of its peak power. The half-pulse length is
the distance light travels during a time equal to one-half pulse
width.
FIG. 4 shows optical power as a function of time for a series of
pulses exiting the circulator when the optical path length is
one-half of the pulse length. FIG. 4 shows that the peak power of
each pulse occurs at times for which the peak powers of adjacent
pulses have fallen to less than one-half of their peak values. This
demonstrates that, even when the optical path is equal to one-half
the pulse length, the optical power is significantly reduced.
The power in each pulse formed by the dielectric beamsplitter 10
depends on the reflectivity of the dielectric beamsplitter 10. The
beamsplitter 10 has a dielectric multi-layer film that is partially
transmitting; absorption and other losses in the beamsplitter are
negligible. If R is the reflectivity of the dielectric beamsplitter
10, then transmissivity T is T=1-R. The optical power of each pulse
is found using the following equations:
wherein E is the power of the pulse emitted by the laser; E.sub.1
is the power of the first output pulse; E.sub.2 is the power of the
second output pulse formed by transmission of the first circulating
pulse through the beamsplitter; E.sub.3 is the power of the third
output pulse formed by transmission of the second circulating pulse
through the beamsplitter; E.sub.n is the power of the nth output
pulse formed by transmission of the (n-1)th circulating pulse.
As shown in FIGS. 3 and 4, the powers of the first and second
output pulses are the largest. As is apparent from the foregoing
equations (1)-(4), the relative magnitudes of the pulses depend on
the reflectivity of the beamsplitter 10. In order to minimize the
maximum optical power of the combined pulses, the reflectivity R of
the dielectric beamsplitter 10 should be chosen so that E.sub.1
(the power of the first output pulse) and E.sub.2 (the power of the
second output pulse) are nearly equal. Using the foregoing
equations (1)-(4), the reflectivity R is determined by the
following equation:
Therefore, the reflectivity R of the dielectric beamsplitter 10
should be about 38.2 percent. With this value of reflectivity R,
the ratio of E (the power of the pulse emitted by the laser) to
E.sub.n (the power in the nth output pulse) is given by the
following equations:
first output pulse E.sub.1 /E = 38.2% (5) second output pulse
E.sub.2 /E = 38.2% (6) third output pulse E.sub.3 /E = 14.6% (7)
fourth output pulse E.sub.4 /E = 05.6% (8) fifth output pulse
E.sub.5 /E = 02.1% (9)
It is difficult to keep the powers of the first and second pulses
exactly equal because the reflectivity R will generally vary
slightly from the ideal value due to manufacturing errors. If the
pulse powers are chosen so that the first and second output pulses
have powers less than 50 percent of the power of the original laser
pulse, then the reflectivity R can have a broad range of values.
Using the foregoing equations (1)-(4) and setting both E.sub.1 and
E.sub.2 to be less than 50 percent of E, the reflectivity R must
satisfy the following conditions:
These inequalities are readily solved to find the appropriate range
of values for the reflectivity R of the dielectric beamsplitter
10:
Furthermore, in the first example embodiment, the laser pulses were
assumed to be linearly polarized light, but they could be
unpolarized, circularly polarized, or randomly polarized light.
However, in these cases, the reflectivity of the dielectric
beamsplitter 10 should be the same for all polarizations. For
example, the reflectivity for both s-polarization and
p-polarization should be equal.
However, even if the s-polarization and p-polarization
reflectivities are different, a similar reduction in power is
obtained if the average of these reflectivities satisfies the
inequality (12). The relative percentage of s-polarization and
p-polarization in the first pulse of light will differ from the
percentage of s-polarization and p-polarization in the second pulse
of light, but the optical power will be reduced.
FIG. 5 shows a block diagram of a pulse-width extending optical
system according to a second example embodiment of the invention.
This embodiment is similar to the first example embodiment, but the
action of the transmissivity and the reflectivity of the half
mirror comprising the means for splitting light and the means for
synthesizing light are reversed. The second example embodiment is
explained below pointing out the differences with respect to the
first example embodiment.
FIG. 5 shows certain features of the second example embodiment. A
laser 1 emits a light pulse that strikes a dielectric beamsplitter
20 comprising a partially reflecting dielectric multi-layer film.
The beamsplitter 20 splits the laser pulse into a first transmitted
pulse and a first circulating pulse. The first transmitted pulse is
output. The first circulating pulse is then sequentially reflected
by mirrors 21, 22, 23, 24. The mirror 24 reflects the first
circulating pulse to the dielectric beamsplitter 20.
The dielectric beamsplitter 20 then splits the first circulating
pulse. The transmitted portion of the first circulating pulse
becomes a second circulating pulse; the reflected portion is a
second output pulse that is directed to the output. The second
circulating pulse follows the same optical path as the first
circulating pulse and is sequentially reflected by the mirrors 21,
22, 23, 24. The mirror 24 reflects the second circulating pulse to
the dielectric beamsplitter 20.
The dielectric beamsplitter 20 then splits the second circulating
pulse into a third output pulse and a third circulating pulse. The
third output pulse is the portion of the second circulating pulse
reflected by the dielectric beamsplitter 20; the third output pulse
is directed to the output. The portion of the second circulating
pulse transmitted by the beamsplitter 20 becomes the third
circulating pulse. The third circulating pulse follows the same
optical path as the first and second circulating pulses and returns
to the dielectric beamsplitter 20.
The dielectric beamsplitter 20 again divides the circulating pulse
into a fourth output pulse and a fourth circulating pulse. The
fourth output pulse is reflected to the output; the fourth
circulating pulse follow the same optical path as the other
circulating pulses before returning to the beamsplitter 20.
As will be readily apparent, there are still more circulating
pulses and output pulses than the four discussed above. The
circulating optical path of the circulator is formed by the
disposition of the four mirrors 21, 22, 23, 24 so as to provide a
predetermined optical-path-length difference or form a pulse-delay
optical path. The mirrors 21, 22, 23, 24 have the function of an
optical system that provides an optical-path-length difference or
forms a pulse-delay optical system. The magnitude of the output
pulses decreases with every reflection by the dielectric
beamsplitter 20 because reflection by the dielectric beamsplitter
20 directs a portion of each circulating pulse to the output. If
the dielectric beamsplitter 20 exhibits no loss and the mirrors 21,
22, 23, 24 are perfect (i.e., 100-percent reflective), then the
energy of the original laser pulse is delivered to the output
without any loss.
In the second example embodiment, the action of the transmission
and reflection of the dielectric beamsplitter 20 is opposite that
of the first example embodiment. In the second example embodiment,
the transmissivity T of the dielectric beamsplitter 20 needed to
make the power of the first output pulse and the power of the
second output pulse equal is equal to the reflectivity R of the
dielectric beamsplitter 10 needed to make the powers of the first
and second output pulses equal. Therefore, the transmissivity T of
dielectric beamsplitter 20 is 38.2 percent.
Similarly, to make the power E.sub.1 of the first output pulse and
the power E.sub.2 of the second pulse of light less than or equal
to 50 percent of the power E of the original laser pulse, the
transmissivity T of the dielectric beamsplitter 20 should satisfy
the following condition:
29.3% <T<50% (13)
As in the first example embodiment, if the pulses of light from the
laser 1 of the second example embodiment are unpolarized,
circularly polarized, or randomly polarized, then approximately the
same effect is achieved if the transmissivity averaged over the s-
and p-polarizations satisfies condition (13), above.
FIG. 6 shows a block diagram of a pulse-width extending optical
system corresponding to a third example embodiment of the
invention. In contrast to the first and second example embodiments
in which a single beamsplitter both divides light pulses and
directs multiple pulses to the output, the third example embodiment
utilizes a first polarizing beamsplitter 30 for splitting light and
a second polarizing beamsplitter 33 for directing light to an
output. The third example embodiment is explained below while
pointing out differences relative to the first and second example
embodiments.
In the pulse-width extending optical system in FIG. 6, a light
pulse emitted by the laser 1 is split into p-polarized and
s-polarized components by the first polarizing beamsplitter 30. The
first polarizing beamsplitter 30 completely transmits p-polarized
light while completely reflecting s-polarized light. The
p-polarized component of the pulse transmitted by the first
polarizing beamsplitter 30 is also transmitted by the second
polarizing beamsplitter 33 and is directed to the output. In
contrast, the s-polarized component of the pulse is reflected by
the first polarizing beamsplitter 30 and is then reflected
sequentially by mirrors 31, 32 before reaching the second
polarizing beamsplitter 33. The second polarizing beamsplitter 33
then reflects the s-polarized pulse portion to the output.
In this way, the first polarizing beamsplitter 30 serves to divide
the input light pulse along two optical paths. The second
polarizing beamsplitter 33 serves to recombine the beams and direct
the recombined beam to the output.
The p-polarized and the s-polarized portions of the original laser
pulse travel different optical paths. As stated above, if this
optical path difference between the two portions is longer than the
laser pulse length, then the two split pulses will have no temporal
overlap at the output. Also, as noted above, even if the optical
path difference is set to one-half the pulse length, the power at
the output is significantly reduced.
Unlike the first and second example embodiments, the original laser
pulse in the third example embodiment is split into two pulses and
each of these pulses becomes one of only two output pulses. There
is no series of output pulses obtained from a series of circulating
pulses.
The third example embodiment splits input pulses that are either
unpolarized, circularly polarized, or randomly polarized into two
pulses having pulse powers that are approximately equal, each
having 50 percent of the power of the input laser pulse.
If the original laser pulse is linearly or elliptically polarized,
then the power in the p-polarized and s-polarized pulses will not
generally be equal. It is desirable to arrange the polarization
axis of the first polarizing beamsplitter 30 so that the powers of
the pulses are nearly equal. If the original laser pulse is
linearly polarized and its polarization orientation is either
parallel or perpendicular to the plane of the page of FIG. 6, then
it is desirable to orient the polarizing axis of the first
polarizing beamsplitter 30 at an angle of 45 from the plane of the
page and perpendicular to the emission direction of the laser
1.
In the third example embodiment, two non-polarizing beamsplitters
can be used instead of the polarizing beamsplitters 30, 33,
respectively. However, in such an alternative case, some optical
power would likely be lost. Referring to FIG. 6, the pulse portion
that would be transmitted by a non-polarizing beamsplitter at 30
generally would not be fully transmitted by a non-polarizing
beamsplitter at 33; a portion of the pulse would be reflected
downward (i.e., toward the bottom of the drawing) away from the
output and thus would be lost. Similarly, the pulse portion
reflected by a non-polarizing beamsplitter at 30 would not be fully
reflected by the non-polarizing beamsplitter at 33; the transmitted
portion would continue toward the bottom of the drawing and thus
would be lost.
FIG. 7 is an optical block diagram of the pulse-width extending
optical system according to the fourth example embodiment of this
invention. The fourth example embodiment is similar to the first
example embodiment, except that the fourth example embodiment has a
halfwave retarder 41 in the optical path of the circulating optical
system. The fourth example embodiment is explained below pointing
out the differences relative to the first example embodiment.
The pulse-width extending optical system used in the FIG. 7
embodiment comprises a laser 1 that emits a nearly linearly
polarized light pulse. For purposes of describing this embodiment,
the direction of polarization of the laser pulses is parallel to
the plane of the page of FIG. 7. Light polarized in this direction
will be referred to as "p-polarized." It will be readily apparent
to those skilled in the art that other polarization directions can
be accommodated.
A p-polarized pulse from the laser 1 enters a dielectric
beamsplitter 40 that splits the pulse into a reflected pulse and a
transmitted pulse. The reflected pulse is reflected by a mirror 14
to an output. The transmitted pulse is reflected sequentially by
mirrors 11, 12 before entering the halfwave retarder 41.
The halfwave retarder 41 is made of an optically anisotropic
material and can rotate the orientation of linearly polarized
light. Linearly polarized light, polarized at an angle with respect
to the axis of a halfwave retarder, exits the halfwave retarder
linearly polarized at an angle--with respect to the axis of the
halfwave retarder. This is equivalent to a rotation of the
direction of polarization by 2. For example, if light enters a
halfwave retarder polarized at +45 degrees with respect to the axis
of the retarder, then the light exits polarized at -45 degrees, or
equivalently, with its polarization rotated by 90 degrees. This and
other properties of halfwave retarders are well-known.
The halfwave retarder 41 is oriented so that its axis is at an
angle of 45 degrees with respect to the plane of the drawing and to
the path of the pulse between mirror 40 and the mirror 13. The
halfwave retarder 41 therefore rotates the direction of
polarization of light passing through it by 90 degrees. Thus, every
pulse entering the halfwave retarder 41 as p-polarized exits as
s-polarized; s-polarized pulses entering the halfwave retarder 41
similarly exit as p-polarized. Therefore, the first circulating
pulse is rotated from p-polarization to s-polarization by halfwave
retarder 41 before the first circulating pulse returns to the
dielectric beamsplitter 40.
The dielectric beamsplitter 40 then transmits a portion of the
first circulating pulse. This portion is the second output pulse.
The mirror 14 reflects the second output pulse toward the output.
The dielectric beamsplitter 40 reflects a portion of the first
circulating pulse, forming a second circulating pulse. The mirrors
11, 12, 13 reflect the second circulating pulse, returning it to
the beamsplitter 40 after the second circulating pulse passes
through the halfwave retarder 41 (which changes the polarization of
the pulse from s-polarization back to p-polarization).
The dielectric beamsplitter 40 transmits a portion of the second
circulating pulse, forming a third output pulse. The mirror 14
reflects the third output pulse to the output. The portion of the
second circulating pulse reflected by the dielectric beamsplitter
40 becomes a third circulating pulse. The third circulating pulse
is reflected by the mirrors 11, 12, 13 and transmitted by the
halfwave retarder 41 which changes the third circulating pulse from
p-polarization to s-polarization and then returns to the dielectric
beamsplitter 40.
In this way the pulses that are reflected by the dielectric
beamsplitter 40 travel the same optical path via the mirrors 11,
12, 13, and the halfwave retarder 41. The light pulses reflected by
the dielectric beamsplitter 40 alternate polarization between
p-polarization and s-polarization. After a pulse has traveled the
optical path, a portion of the pulse is transmitted to the output
by the dielectric beamsplitter 40. The magnitude of a circulating
pulse declines with each reflection by the dielectric beamsplitter
40 because a portion of the pulse is transmitted to the output.
Eventually, the energy in the original laser pulse is delivered to
the output.
The power of the circulating and output pulses depends upon the
reflectivity of the dielectric beamsplitter 40. The dielectric
beamsplitter 40 generally has different reflectivities for
s-polarized and p-polarized pulses. For purposes of explanation,
the reflectivity of the dielectric beamsplitter 40 to p-polarized
pulses is R.sub.p and the reflectivity to s-polarized pulses is
R.sub.s. The powers E.sub.1, E.sub.2, and E.sub.3 of the first
three output pulses is given by equations (14)-(16):
In the fourth example embodiment, the halfwave retarder 41 is
placed in the optical path of the circulating optical system. The
halfwave retarder 41 alternately changes the polarization of the
pulses so that the power E.sub.1 of the first output pulse, the
power E.sub.2 of the second output pulse, and the power E.sub.3 of
the third output pulse can be made early equal. If equations
(14)-(16) are solved such that E.sub.1 =E.sub.2 =E.sub.3, then the
reflectivities R.sub.s and R.sub.p required may be determined.
The solution is R.sub.p =29.3% and R.sub.s =58.6%.
With these reflectivities, the powers E.sub.1, E.sub.2, and E.sub.3
of the first three output pulses are approximately 29.3 percent of
the power E of the original laser pulse. In this way, the fourth
example embodiment achieves lower peak power in the output pulses
than the first example embodiment in which the powers E.sub.1 and
E2 of the first two output pulses are approximately 38.2 percent of
the power E of the original laser pulse.
It is also possible to provide for errors in the reflectivities of
the dielectric beamsplitter 40. For example, the reflectivities can
be selected such that the powers E.sub.1, E.sub.2, and E.sub.3 of
the first three output pulses are 40 percent or less than the power
E of the original laser pulse. Using the foregoing equations
(14)-(16) to determine the magnitudes of E.sub.1, E.sub.2, and
E.sub.3, the reflectivities R.sub.s and R.sub.p must satisfy the
following inequalities:
It will be readily apparent that, if the original laser pulse is
s-polarized instead of p-polarized, interchanging R.sub.s and
R.sub.p in expressions (14)-(19) gives the correct expression.
FIG. 8 is an optical block diagram of a pulse-width extending
optical system according to a fifth example embodiment of the
invention.
The pulse-width extending optical system of the fifth example
embodiment is similar to that of the second example embodiment of
FIG. 5, differing primarily in the placement of a halfwave retarder
51 in the optical path of the circulating optical system. In the
fifth example embodiment, as in the fourth example embodiment, the
powers E.sub.1, E.sub.2, and E.sub.3 of the first three output
pulses can be made nearly equal. The fifth example embodiment is
explained below pointing out the differences relative to the second
and fourth example embodiments.
A laser 1 emits a p-polarized laser pulse. As defined above,
p-polarization is the polarization direction in the plane of the
page of FIG. 8 and perpendicular to the direction of propagation of
the laser pulse. The equations describing the magnitudes of the
output pulses are readily found by substituting T.sub.s and T.sub.p
for R.sub.s and R.sub.p, respectively, in the equations pertaining
to the fourth example embodiment. T.sub.s and T.sub.p are the
transmissivities of a dielectric beamsplitter 50 for the s- and
p-polarized pulses, respectively. This is apparent by noting that
the first output pulse of the fifth example embodiment is
transmitted by the dielectric beamsplitter 50 and subsequent output
pulses are formed by reflection of circulating pulses. In contrast,
in the fourth example embodiment, the first output pulse is
reflected by a beamsplitter and subsequent output pulses are the
transmitted portions of circulating pulses. Therefore, the powers
E.sub.1, E.sub.2, and E.sub.3 of the first three output pulses are
equal if the dielectric beamsplitter 50 has transmissivities
T.sub.p =29.3 percent and T.sub.s =58.6 percent.
It will be readily apparent that, if the original laser pulses are
s-polarized, the transmissivities T.sub.p and T.sub.s of the
dielectric beamsplitter 50 should be interchanged.
FIG. 9 is an optical block diagram of a pulse-width extending
optical system according to the sixth example embodiment of the
invention. The pulse-width extending optical system of the sixth
example embodiment is similar to that of the second example
embodiment as shown in FIG. 5, differing primarily from the second
example embodiment in the placement of a relay system 61, 62
(preferably a Keplerian relay system) in the optical path of the
circulating optical system. The sixth example embodiment is
explained below pointing out the differences relative to the second
example embodiment.
As shown in FIG. 9 the relay system 61, 62 is placed in the optical
path of a circulating optical system formed by mirrors 21, 22, 23,
24. The relay system 61, 62 controls the tendency of the beam
cross-section of the laser pulses to increase. As is well-known,
beams of light naturally diverge. The relay system 61, 62 is
arranged so that the beamsplitting surface of a dielectric
beamsplitter 20 and the beam-combining surface of the dielectric
beamsplitter 20 are conjugate, i.e., the beamsplitting surface and
the beam combining surfaces are imaged onto each other. In general,
surfaces imaged onto each other are conjugate surfaces.
When the laser 1 has a large beam divergence such as is common with
excimer lasers, the circulating pulses tend to diverge and spread
out as they propagate around the circulating optical system.
FIG. 10 illustrates the divergence of the circulating pulses. FIG.
10 shows the intensity of the pulses as function of a coordinate
perpendicular to the propagation direction. As shown in FIG. 10,
the first pulse is narrowest; the second pulse is wider than the
first pulse and narrower than the third pulse. It is apparent that
the cross-section of pulses gradually enlarges as the pulses
propagate.
The relay system 61, 62 confines the pulses and prevents them from
spreading out, regardless of the number of times they have
propagated through the circulating optical system. Furthermore, the
magnification of the relay system 61, 62 is set so that the first
pulse of light and the second pulse of light are rotated just 180
degrees relative to each other before the beamsplitter 20 directs
them to the output.
As shown in FIG. 11, even if an excimer laser emits a pulse having
a non-uniform cross-section, the odd-numbered pulses and the
even-numbered pulses are rotated 180 degrees relative to each other
and directed to the output as shown in FIG. 12. In this way, not
only is beam-spreading controlled but the optical power delivered
to the output is delivered with a more uniform distribution.
In the sixth example embodiment of FIG. 9, the optical path joining
the laser 1 and the dielectric beamsplitter 20 crosses the optical
path joining the mirrors 22 and 23. If it is undesirable to have
these paths cross, crossing may be avoided by arranging the optical
path formed by the mirrors 21, 22, 23, 24 and the relay system 61,
62 in a plane other than the plane of the page of FIG. 9.
Furthermore, it is generally desirable that the relay system 61, 62
have a magnification of either 1 or -1. The focal point is at the
mid-point of the relay system, so optical elements (such as the
mirrors 21, 22, 23, 24) are preferably sufficiently distant from
the focal point so that they are not damaged by the high light
intensity at the focal point.
FIG. 13 is an optical block diagram of a pulse-width extending
optical system according to a seventh example embodiment of the
invention.
The pulse-width extending optical system used in the seventh
example embodiment is similar to that of the first example
embodiment of FIG. 1, differing primarily from the first example
only in the placement of a relay system 71, 72 (preferably
Keplerian) in the optical path of the circulating optical system.
In the seventh example embodiment, as in the sixth example
embodiment, the beamsplitting surface of the beamsplitter 10 and
the beam combining surface are nearly conjugate by means of the
relay system 71, 72 which preferably has a magnification of -1. In
this example embodiment, both beam divergence and beam uniformity
are improved.
FIG. 14 is an optical block diagram of a pulse-width extending
optical system according to an eighth example embodiment of the
invention.
The pulse-width extending optical system of the eighth example
embodiment is similar to that of the third example embodiment,
differing from the third example embodiment primarily in the
placement of a relay system 81, 82 (preferably Keplerian) in the
longer of the two optical paths. In the eighth example embodiment,
as in the sixth and seventh example embodiments, the beamsplitting
surface of the first beamsplitter 30 and the directing (combining)
surface of the directing component are nearly conjugate by means of
the relay system 81, 82 which has a magnification of -1.
FIG. 15 is an optical block diagram of a projection-exposure
apparatus according to a ninth example embodiment of the invention,
the apparatus comprising a pulse-width extending optical system as
described above.
The projection-exposure apparatus of the ninth example embodiment
comprises an excimer laser 60 that emits laser pulses. The laser
pulses emitted by the excimer laser 60 enter a dielectric
beamsplitter 20 through a relay system 91, 92 (preferably
Keplerian). The relay system 91, 92 is configured so that the
output aperture of the excimer laser 60 and the beamsplitting
surface of the dielectric beamsplitter 20 are nearly conjugate.
The magnification of the relay system 91, 92 depends on the size of
the dielectric beamsplitter 20 and the beam cross-section of the
laser emission. Even if the laser 60 emits pulses at an angle to
the optical axis of the relay system 91, 92 and not along the axis,
the relay system 91, 92 still guides the pulses from the laser 60
to the dielectric beamsplitter 20.
As in the pulse-width extending optical system in the sixth example
embodiment shown in FIG. 9, the pulses entering the dielectric
beamsplitter 20 shown in FIG. 15 are split into pulses transmitted
by the dielectric beamsplitter 20 (output pulses) and pulses
reflected by the dielectric beamsplitter 20 (circulating pulses).
The mirrors 21, 22, 23, 24 define a circulating optical path; the
circulating optical path comprises a second relay system 93, 94.
The circulating return to the dielectric beamsplitter 20 after
propagating around the circulating path. The circulating pulses
return to the dielectric beamsplitter 20, and portions are
reflected by the beamsplitter 20. The output portions are generally
directed along the same optical path. The relay system 93, 94
preferably has a magnification of -1 and is configured so that the
beamsplitting surface of the dielectric beamsplitter 20 (which acts
as a both a splitting mechanism and an output mechanism) is
conjugate to itself with respect to the circulating path, i.e., the
relay system 93, 94 images the beamsplitter onto itself with a
magnification of -1.
The relay system 91, 92, the dielectric beamsplitter 20, the
mirrors 21, 22, 23, 24, and the relay system 93, 94 form a
pulse-width extending optical system for the laser 60. The
pulse-width extending optical system of the projection-exposure
apparatus of FIG. 15 also prevents beam spatial divergence with the
relay system 93, 94. As discussed previously, this relay system
improves the spatial uniformity of the beam as well.
If the optical path length of the circulating optical system is
larger than one-half the pulse length, then the pulse-width is
extended and the peak power is reduced. As a result, the
performance degradation of optical elements and systems following
the pulse-width extending system is reduced and component life is
extended.
The output pulses of this pulse-width extending system are directed
along a common optical path through the dielectric beamsplitter 20
and to a third keplerian relay system 95 and 96. A mirror 97 then
reflects the pulses into, for example, a beam-correcting optical
system 2 comprising both cylindrical and spherical lenses. This
beam-correcting optical system 2 changes the beam cross-section;
typically, elliptical cross-sections are reshaped to be more nearly
circular.
The corrected beam enters a fly-eye lens 3. The fly-eye lens 3, as
known in the art, has multiple lens elements arranged in parallel
along the optical axis of the optical system. Pulses incident to
the fly-eye lens 3 are formed into several secondary images
(secondary light sources) at the rear focal point of the fly-eye
lens 3. (The secondary images of the incident radiation received by
the fly-eye lens 3 produce more uniform irradiation.)
The beam-correcting optical system 2 is arranged so that the
surface of the fly-eye lens 3 into which the radiation enters and
the dielectric beamsplitter 20 are nearly conjugate. Even where
there are slight angular differences between the optical axis of
the illumination optical system and the exit direction of the
output pulses from the dielectric beamsplitter 20, the light pulses
from the beamsplitter 20 are accurately guided to the entrance
surface of the fly-eye lens 3.
If a cylindrical lens is included in the beam-correcting optical
system 2, as indicated in the detailed summary and drawings of U.S.
patent application Ser. No. 08/603,001 (filed on Feb. 16, 1996;
incorporated herein by reference), it is desirable to have the
dielectric beamsplitter 20 and the entrance surface of the fly-eye
lens 3 configured so that they are nearly conjugate in both
horizontal and vertical directions.
Light from the multiple secondary light sources formed by the
fly-eye lens 3 is limited by aperture 4 and is focused by a
condenser lens 5. The condenser lens 5 directs the light to a
mirror 98 that reflects the beam to a mask 6 that is thereby nearly
uniformly irradiated. The mask 6 contains patterns such as those of
a semiconductor integrated circuit. A projection optical system 7
projects the pattern of the mask 6 (as reduced or enlarged) onto a
wafer 8 that has been coated with a resist sensitive to the
radiation from the laser. Radiation from the laser exposes the
resist coating on the wafer 8. After being exposed in the
projection-exposure apparatus shown in FIG. 15, the wafer 8
undergoes further processing, including development and etching, in
which all but the resist pattern are removed. Following the etching
process, other processes such as a resist-removal process are
performed to conclude the process. After the wafer 8 is fully
processed, it is diced (cut into chips), bonded (wires are
attached), and then packaged. The chip is then ready to use.
The foregoing example refers to the manufacture of large scale
integration (LSI) semiconductor devices, but projection-exposure
apparatus according to the present invention can be used in other
manufacturing processes. For example, the projection-exposure
apparatus can be used in the manufacture of liquid-crystal-display
elements, thin-film magnetic heads, imaging elements (such as
CCDs), and other semiconductor devices.
The laser 60, the pulse-width extending optical system, the relay
system 95, 96, the beam-correcting optical system 2, the fly-eye
lens 3, the aperture 4, the condenser lens 5, and the turning
mirror 98 form an illumination system that illuminates the mask 6
spatially uniformly and with reduced peak optical power. By
splitting the laser pulses into multiple pulses, the total laser
power delivered to the wafer 8 is (neglecting losses) the same as
the total power delivered without splitting the pulses. Splitting
the laser pulses and extending the laser pulse-width reduces the
peak optical power, reducing degradation of optical elements.
In the ninth example embodiment shown in FIG. 15, the
beam-correcting optical system 2 is situated upstream of the
fly-eye lens 3. However, the beam-correcting optical system 2 may
be in other locations. As stated above, as long as the laser 60,
the dielectric beamsplitter 20, and the fly-eye lens 3 are
conjugate, it is possible, for example, to place the
beam-correcting optical system 2 directly behind the laser 60 or in
the optical path between the dielectric beamsplitter 20 and the
lens 95 of the relay system 95, 96.
Whereas the invention has been described in connection with
multiple example embodiments, it will be understood that the
invention is not limited to those embodiments. On the contrary, the
invention is intended to encompass all alternatives, modifications,
and equivalents as may be included within the spirit and scope of
the invention as defined by the appended claims.
* * * * *